CHEMISTRY AND PHTSIG8 OF PLANTS. 79 



151. How the Food is Transported in the Plant. — Once 

 within the plant-body, the food-materials diflEuse to all 

 watery parts, in the case of the larger terrestrial plants 

 rising through the stem to the leaves. By diffusion there 

 is a constant tendency toward an equal distribution through- 

 out the plant of the solutions which enter it; and if there 

 were no disturbing chemical reactions taking place, such a 

 condition would in most plants be soon reached. It is quite 

 probable, indeed, that this actually happens for certain 

 substances which are found in solution in the soil or water, 

 and which, entering plants, diffuse through them to all 

 parts, but not being used they soon reach a state of equal 

 diffusion, which is only slightly disturbed by the extension 

 of the plant-body by growth. The diffusion of food-mate- 

 rials throughout terrestrial plants is aided by the evapora- 

 tion of water from the leaves, thus causing a strong upward 

 movement of tlie water which contains the various solutions 

 of food-matter; but it is not dependent upon evaporation, 

 for diffusion takes place under conditions which preclude 

 evaporation. 



152. Starch-making, or Assimilation. — Most of the food- 

 materials of plants can be directly used by the protoplasm. 

 Thus the oxygen and water, and the nitrates, sulphates, 

 etc., mentioned above may be at once made use of by the 

 protoplasm for its own nourishment. It is not so, however, 

 with the carbon dioxide. It cannot be used directly as 

 food, but must first undergo a special preparation. It must 

 be broken up and recombined along with the elements de- 

 rived from water so as to form a new compound which the 

 protoplasm can digest. This new compound is starch, or 

 something much like it, and so we may call this prepara- 



